Shulou(Shulou.com)06/01 Report--

In-depth Analysis of RMAN-- backup data blocks

For RMAN backup is based on block backup, the principle of block backup is that RMAN compiles the list of files to be backed up based on the backup algorithm rules. Based on the number of channels and the number of files backed up at the same time, RMAN creates some memory buffers in the Oracle shared memory segment. It's usually within PGA, but sometimes memory buffers are pushed into SGA. The channel server process then starts reading the data files and populating the blocks in the RMAN buffer. When a buffer is filled, blocks in the input buffer are pushed into the output buffer. This memory-to-memory memory-to-memory write occurs for each block in the data file, and the block remains in the output buffer until the buffer is filled. Once the output buffer is filled, the contents of the output buffer are pushed to the backup location (disk or tape)

Once all files have been filtered through the memory buffer, the backup slice is completed, and RMAN writes the completion time of the backup slice and the name of the backup slice in the target database control file.

Advantages of block backup:

A memory-to-memory write operation occurs when each data block is read from disk to memory, during which the corruption of the data block is checked.

Empty block compression:

RMAN can choose empty block compression (null block compression) when accessing blocks, delete blocks that have never been used (header is 0), and discard those blocks when performing memory write operations to memory. Therefore, we only need to back up the used blocks.

Unused block compression:

Uninitialized blocks will not be backed up, and even used blocks will not be backed up after version 10.2.0.3 if they are empty.

Unused block compression can be used automatically, but the following conditions need to be met:

Full backup or level 0 backup is required during backup

Backup is disk backup (or Oracle security backup)

The initialization parameter of COMPATIBLE is set to 10.2 or higher

There is no reliable restore point for the database to be backed up

Data files are managed locally

Performance of block-level backups:

Hot backup mode (begin backup) does not need to be started during RMAN backups. RMAN's access to the data block allows RMAN to coordinate with the DBWR process that writes the "dirty" buffer, and it can wait until the data block is consistent before reading the block into memory. Therefore, the blocks are not dumped for redo, and the blocks in the backup are always consistent.

Block tracking for RMAN:

Block change tracking records the changes in data blocks since the last backup and records this information in a trace file. RMAN uses this file to determine the changed data that needs to be backed up in an incremental backup. This greatly improves backup performance, and RMAN can no longer scan the entire file to find change data. For this reason, Oracle introduces a new background process, CTWR, whose full name is Change Tracking Writer, which is used to record the changed blocks and write the changes to the corresponding log files. Track the change of data blocks between two backups through bitmaps; switch bitmaps before each backup; consider the limit of 8 bitmaps when developing incremental backup strategy; generally, incremental backup cycle is one week in RAC environment, change tracking file needs to be placed on shared storage; the size of Change tracking file is proportional to the size of database and the number of redo thread of enabled; the size of Change tracking file is independent of the frequency of data updates. Turning on block tracking has a slight impact on database performance. [oracle@aix211] $sqlplus'/ as sysdba'SQL*Plus: Release 10.2.0.1.0-Production on Mon May 19 17:06:05 2014Copyright (c) 1982, 2005, Oracle. All rights reserved.Connected to:Oracle Database 10g Enterprise Edition Release 10.2.0.1.0-64bit ProductionWith the Partitioning, OLAP and Data Mining optionsSQL > ALTER DATABASE ENABLE BLOCK CHANGE TRACKING USING FILE'/ home/oracle/block.trc';Database altered.SQL > SELECT filename from V$BLOCK_CHANGE_TRACKING FILENAME----/home/oracle/block.trc

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.